With the Combat Zone facing demolition in 1970, a graduate student in architectural history set out to survey the neighborhood. Peering above adult bookstores and X-rated cinemas, Cynthia Zaitzevsky admired the forlorn upper stories of several 19th-century buildings, but one stopped her in her tracks.

Grime obscuring its salmon-colored sandstone, the Hayden Building at 681-683 Washington St. remained a fivestoried thing of beauty. At once slender and substantial, its design suggested Henry Hobson Richardson - the best-known American architect of his day and the most celebrated in Boston’s history. But Zaitzevsky knew Richardson’s works had been catalogued, and this wasn’t one of them.

Except it was. Zaitzevsky would unearth an 1875 building permit bearing Richardson’s name and discover that the architect designed the Hayden for his wife’s family, while completing Trinity Church, but never recorded it in his office books. That finding contributed to the defeat of the urban renewal project to raze the area and led to landmark status for the Hayden, but it brought no immediate glory to the decaying building, occupied by a peep show and gay bathhouse. A 1985 fire ravaged the upper floors, leaving the building charred and vulnerable.

Now, a long-stalled campaign to save the vacant Hayden Building is barreling toward the finish, with $5.8 million in funding and a plan not to freeze the Hayden in amber but resuscitate it. Richardson’s last remaining commercial building in Boston - considered a prototype for the modern skyscraper - will be revitalized with first-floor retail and four floors of apartments. Historic Boston Inc., which championed the Hayden’s rebirth, will hold a groundbreaking with Mayor Thomas M. Menino to celebrate Monday.

“This is true preservation by standing in front of the wrecking ball,’’ said Kathy Kottaridis, executive director of the nonprofit Historic Boston, which has kept the building on life support since acquiring it two decades ago. “Though in this case, the wrecking ball was benign neglect.’’

In 1990, the Hayden was listed by American Heritage magazine as one of the dozen greatest American buildings in danger of disappearing, a year after the city threatened legal action to compel the Hayden’s anonymous out-of-town ownership trust to board it up.

The Combat Zone then was still crammed with licensed and unlicensed adult entertainment. LaGrange Street, the one-way block running along the north side of the Hayden Building, was the main drag plied by prostitutes.

Standing before that bombed-out building in the red-light district, one could scarcely have imagined that it might one day boast airy, floor-through apartments - two-bedroom, 1 1/2-bath units with exposed brick and modern amenities, that will be marketed for at least $3,500 a month when finished early next year.

In a statement, Menino, said the Hayden project “preserves a gem in our city and continues the renaissance of Boston’s Midtown Cultural District,’’ an optimistic name coined by planners in the 1980s and only now coming to fruition. The Combat Zone has been scrubbed out by forces large and small - home entertainment, the Internet, the boom in urban living, the determination of officials, activists, and developers. Two lonely strip clubs, tucked behind the Hayden, are all that remain.

Up Washington, three classic theaters - the Opera House, Modern, and Paramount - are lit up again. A host of recently built, under-construction, or planned high-rises fan out around the Hayden: thousands of units, billions in investment. Immediately across LaGrange, a 29-story, $170 million residential tower is underway.

The Hayden, by comparison, is a speck: 62 feet tall, filling 1/30th of an acre. But it boasts what Globe architecture critic Robert Campbell dubbed “the most muscle-bound architecture’’ of any building in Boston, inch for inch.

“It’s a small building, kind of out of the way, but really a very important building in terms of Richardson’s own career,’’ said James F. O’Gorman, a Wellesley College professor emeritus who has written extensively on Richardson, the first prominent American architect not to parrot Europe but to forge his own style, known as Richardsonian Romanesque.

Best known for churches, libraries, and train stations, Richardson’s scattered commercial buildings were no less significant. His Marshall Field’s Wholesale Store in Chicago - razed in the 1930s - influenced Louis Sullivan and Frank Lloyd Wright; its understated, elevated design pushed the limits of load-bearing stone and inspired the first steel-framed skyscrapers. The Hayden, long unrecognized, represents an earlier iteration, O’Gorman said.

Richardson designed it for the estate of his late father-in-law, John C. Hayden, on a plot vacated by a fatal explosion that flattened a drugstore. He partnered with Norcross Brothers - the same firm in the midst of erecting Trinity Church - and used coveted Longmeadow sandstone for the facade, a material also employed for Trinity.

The original interior, long lost, was likely unspectacular; the building’s calling card was its shell. When Historic Boston bought it for $425,000 in 1993 - along with a less historic neighbor, the empty Intermission Lounge strip club - it marked a huge sum for the property, the neighborhood, and the nonprofit.

By then, Historic Boston had saved and reclaimed dozens of historic, culturally significant buildings, but none posed as daunting or expensive a challenge as the Hayden. Kottaridis called it the organization’s “most profound labor of love’’ since the campaign that gave rise to Historic Boston: the 1960 effort to save the Old Corner Bookstore, a 1718 structure that was slated to become a parking garage but is now a Freedom Trail landmark.

The market 20 years ago could not justify the investment needed to fully redevelop the Hayden.

So Historic Boston invested more than $1 million to restore the rich salmon sandstone and arcade windows of the facade, replace a roof open to the sky, and reinforce walls against collapse.

It also lured Penang, a Malaysian restaurant, to take the place of the former strip club and, for a few years, a bank for Hayden’s ground floor, beneath the husk of its charred, empty upper stories. And it waited, tending other projects while values in the old Combat Zone soared.

Last year, Historic Boston split off and sold the Penang building for $2.9 million, generating capital for the Hayden and other redevelopments. State and federal preservation credits will cover $2 million. The city provided a $250,000 low-interest loan, and bank loans fund the rest, to be repaid with rental income and a new fund-raising campaign seeded with $100,000 from developer and philanthropist Ronald Druker, Kottaridis said.

The result, designed by Boston’s CUBE architects and being built by Cambridge contractor Marc Truant & Associates, will modernize the interior while celebrating Richardson’s architecture (notably the ample windows that give the stone building unusual light and transparency) and Norcross’s construction, exposing not just structural brick but some of the footholds for the 1875 scaffolding.

Displays in the stairwell will tell the story of Richardson and the building, with its varied roster of tenants across the decades: tailors, jewelers, engravers, printers; a dental parlor advertising $6 false teeth and innovative cocaine-free dentistry “without the least particle of pain or danger’’; a uniform company and an employment service; a record shop; an Army-Navy store.

But they will not sweep away the Combat Zone past. Though contractors have already carted off dumpsters full of debris, they spared something from the recent past: dozens of dusty reels of adult films that had been stashed in the building.

The celluloid will be woven into an artistic screen for the lobby that will be visible from outside, where Richardson stood, Zaitzevsky paused, and street walkers once plied their trade.